Embryonic tissues may be dissociated by mild proteolysis or other treatments to yield suspension of single cells that will adhere, aggregate and eventually produce many of the features of the original tissue. When aggregates are prepared from cells derived from two different tissues, these cells will eventually segregate into two homotypic aggregates. Many studies have suggested that the cell surface glycoproteins may constitute the recognition sites in specific cell-cell interaction. We have succeeded in isolating three different types of cell surface proteins from chick embryo retina, cerebrum, cerebellum, kidney, heart and lung. The high molecular weight cell associating proteins with molecular weight 100,000 enhance selectively long-term cell aggregation and short-term cell adhesion of homologous cells at 0.02 microgram/ml. The low molecular weight cell associating proteins, with molecular weight 3,000, enhance selectively only long-term cell aggregation of homologous cells at 0.02 microgram/ml. The carbohydrate binding proteins have also been isolated at specific developmental ages in various embryonic tissues. The three activities have been separated by gel chromatography, extraction and other preliminary purification methods. We propose to purify and chemically characterize these proteins from two different tissues for structural comparison. The proposed experiments aim at the elucidation of the molecular basis for cell recognition. The results of these experiments should provide better understanding of direct cell-cell interaction in terms of cell association, regulation in cell proliferation and cell differentiation.